accessing parts of memory of a void pointer

I'm using a void pointer to store and copy data into a bunch of arrays. I am trying to make it so that if an array fills up, I continue writing the remaining data into a new array.
For example, I have a 100 byte void pointer. I want to write all of it into an array. The array only has room for 50 bytes so I then want to write the remaining 50 into a different array with available space.
Is it possible to 'index' the data of the void pointer so that I don't reuse the first 50 bytes and can access the last 50 bytes?